Frequent business traveler Geoff Slater says he was a top-tier elite member of Delta Air Lines' frequent-flier program but now tries to avoid flying on Delta.

"I had accrued over 700,000 miles on Delta, but then found they wanted double or triple miles for most everything," Slater says.

Slater, a public transit consultant in Fayston, Vt., is one of many former and current elite members of Delta Air Lines' SkyMiles program who say they're disgruntled because they haven't been able to redeem miles at the minimum mileage level for a free ticket.

Like several other U.S. airlines' frequent-flier programs, the SkyMiles program offers a free round-trip coach ticket within the continental USA for 25,000 miles.

Many seasoned fliers, though, say it's virtually impossible to cash in that amount for a desired flight and one instead must cash in 40,000 miles, Delta's "medium" level award, or 60,000 miles, the "high" award.

Fliers with competing airlines have raised similar complaints about finding a free ticket at a minimum mileage level. But frequent fliers and other business-travel experts say it's most difficult at Delta.

"It's getting very hard on Delta to find a seat for minimum miles," says Aik-Meng Tan, vice president of sales for OKT Travel, a corporate travel management company in Plainview, N.Y. "Of all the mileage programs out there, Delta is the most unfriendly in terms of the number of miles you need to redeem an award ticket," Tan says.

Delta stands behind its frequent-flier program, emphasizing that many components are beneficial for members. Spokesman Paul Skrbec says an advantage of the program is its three mileage redemption levels, unlike two at other airlines.

Comparison shopping

To examine the issue, USA TODAY chose 50 random flights on Delta's website; 42 of them had no free tickets available at the minimum mileage level.

Included were some flights to unpopular wintertime destinations for the ordinarily slow travel season next January. But finding a single free ticket at the minimum-award level was hard even in those instances.

For instance, there were no minimum-level free tickets available for an Atlanta to Fargo, N.D., itinerary, leaving Atlanta on Jan. 13, 2014, and returning Jan. 17. That also was the case for a Reno to Fargo flight on the same dates.

On those dates, a free New York to Los Angeles round-trip ticket was available for 25,000 miles for someone who wouldn't mind a flight arriving in Los Angeles at 12:20 a.m. on Jan. 14. For a flight arriving earlier, 32,500 miles were required.

USA TODAY found minimum-award availability better when it chose 50 random international flights. But it still found no free coach tickets for the minimum miles on 36 of them.

The minimum amount required for a free round-trip coach ticket varies for international flights: 60,000 miles for Europe and more for many other countries.

Some free international tickets available at the minimum level may be impractical. For instance, a round-trip Charlotte to Paris itinerary in September for 60,000 miles has stops in New York and Amsterdam, and a nine-hour wait for outbound connecting flights.

Murray Cook, president of a sports venue management company, says he couldn't get free seats at the minimum level for a Delta flight to Munich in December, though he went online when the flights went on sale — 331 days before departure.

Michael Sommer, a consultant and a Diamond Medallion elite member with more than 2.3 million SkyMiles, is frustrated.

"Why should I have to spend twice as many miles for the same free ticket after earning those miles by flying on so many paid flights?" Sommer says.

After being unable to cash in the minimum number of miles for free business-class tickets for numerous flights departing over an eight-month period, Sommer wrote a complaint letter to Delta CEO Richard Anderson.

In a response letter that Sommer provided to USA TODAY, Delta executive assistant Susanna Curtis apologized for "any frustration" he encountered, pointed out the benefits of the SkyMiles program and said the number of seats offered for free travel is limited.

"Further, I would also like point out that one of the reasons we award Medallion members a mileage bonus (1.25 miles for every mile flown) is so the additional miles make it easier to use the highest award option if the other award levels are not available," Curtis wrote.

Hard to find

Brian Kelly, a frequent-flier expert who runs ThePointsGuy.com website, says "using SkyMiles can be an extremely frustrating and time-consuming experience — even for the most advanced mileage expert."

Delta's Skrbec wouldn't say "yes" or "no" when USA TODAY asked whether every flight when it first goes on sale has at least one free seat available at the lowest mileage level.

"Delta offers every seat as an award seat with redemption levels varying based on route, date of travel and customer demand," the airline's statement reads. "As the price of airfares increase, customer demand for award travel also increases, which can result in availability at the lowest levels being sold out."

Jay Sorensen of IdeaWorks, a consulting company specializing in airline revenue, says he suspects many Delta flights have no free seats available at the minimum-mileage level. In a May 2012 survey, Sorensen ranked Delta last of 23 airlines in free seat availability.

Delta says its frequent fliers last year redeemed more than 262 billion miles for more than 11 million awards. "In 2012," the airline says, "8% of revenue miles flown on Delta were for award travel."